Tuning the Carter Thermoquad Carburetor - TQ Dyno Dial Up!
Supertuning The TQ
From the January, 2005 issue of Mopar Muscle
By Steve Dulcich
Photography by Steve Dulcich
Dyno operator Tom is ready...
Dyno operator Tom is ready to demonstrate the improper way to tune a Thermo-Quad carb.
While the AFB and even the AVS carbs get most of the fame, the Carter Thermo-Quad was probably used on more four-barrel Mopar applications than all other carburetors combined. Debuting on the '71 340 (though with a unique metering system in that year) from 1973 until the end of production in 1985, if it was a four-barrel Mopar engine, it had a TQ on top. Interestingly, even among Mopar fanatics, the TQ seems to inspire either wild admiration or open contempt. One thing everyone can agree on is the TQ is king when it comes to flow-per-dollar spent. Basically, in production versions there were two sizes-big and bigger. The small TQ carries 1 3/8-inch primaries, while the larger version measures a substantial 1 1/2-inch. Both share secondary caverns specing out at 2 1/4-inch. That's nearly eight square inches of throttle bore area on the secondary side alone, providing room enough to pass 800 cfm through the smaller version of the carb, and a breezy 850 cfm through the large example.
That's quite a bit of carb, but the TQ seems to defy convention in terms of airflow relative to displacement, since the factory routinely bolted the larger of the two to smog laden 318s. The TQ was hands down the most sophisticated of the Carter four-barrels and the most tunable. For any carb to survive as an OE unit well into the advent of fuel injection and stringent emissions standards, it had to be a precise fuel-metering device. Part of the TQ's secret is in its contoured air door, unlike the flat plate air door used in the AVS, which itself was a quantum leap in technology over the AFB's crude counterweighted velocity valves. The TQ's air door is contoured to create a highly effective venturi in the region of the secondary fuel discharge nozzles. The TQ's sophistication may well have been its undoing as far as universal popularity goes. Just the air door itself can be manipulated by no less than five different settings and adjustments, all of which play a role in how the secondaries react and affect the fuel curve. Add in the tuning permutations possible with three-step metering rods, primary and secondary jets, step-up piston position and tension, accelerator pump timing, stroke and volume, main and auxiliary air bleed circuits, and you can see that the potential to really screw one up is incalculable. Compare that to the most popular carbs, where tuning for most enthusiasts requires just unscrewing a jet and replacing it with one having a bigger number for more gas. Learn the TQ's secrets, however, and these dirt-cheap units can run with the best of 'em.
We recently rebuilt a small TQ for a buddy and wanted to put in some dyno time tuning it to perfection. Glendora Dodge counterman James Schagel was kind enough to donate his time and the use of his Duster for our tuning effort. James' 360-powered A-body was equipped with a conventional high-performance aftermarket square bore carb. Although the object of this study was to delve into the tuning intricacies of the ThermoQuad, we also tuned his carb as part of the deal, which provided us with a mental benchmark of the TQ's performance. Right off the bat, the TQ was performing on par. With both carbs tuned, the TQ actually showed 233 versus 229.7 with the "performance" carb. However, the TQ did have an adapter plate under it that functioned as a carb spacer, adding another variable. One thing is certain, a TQ built and tuned correctly can perform.
Anyone see the makings of...
Anyone see the makings of a sound high-performance induction here? For a budget conscious street application, a $15 Thermo-Quad, a $22 rebuild kit, and a little know-how can add up to a helluva $37 carburetor. This one is a 9134S, a '78 vintage California emissions example of the small factory TQ with a not-so-small 800-cfm rating.
Dissecting a TQ is not difficult,...
Dissecting a TQ is not difficult, as long as it's done in the correct sequence. Start by removing the metering rods. To clear the way, the linkage rod anchor at the choke countershaft is freed by removing a tiny bolt, and then the retainer is removed.
The metering rods hang by...
The metering rods hang by a central yoke, an arrangement as sturdy and reliable as the Brooklyn Bridge. Some TQs require removing the two cover-plates on each side to clear the rods. Two tiny, easy-to-lose screws hold these in place.
Ten screws hold the carb's...
Ten screws hold the carb's three main body-components together-eight of which are visible; two of which are hidden behind the choke. If you ruin your TQ by trying to pry it apart with these two bolts still fastened and crack the phenolic housing, you can save face by claiming the plastic body is warped and needs replacing. This trick has worked for years. Also remove the linkages from the choke pull off, fast idle cam, and accelerator pump.
The body of the TQ consists...
The body of the TQ consists of the throttle body, phenolic fuel bowl assembly, and upper air horn (neatly sandwiched to create a fuel mixer). Strip each subassembly for cleaning. The phenolic fuel bowl carries the primary jets and a sheetmetal diffuser baffle, plus a couple of O-rings lying in the wells next to the primary bores.
The manly secondary jets on...
The manly secondary jets on a TQ use a 5/16-inch wrench to remove.
TQ Factory Metering Rods and Jets
Factory metering rods are all numbered, but the problem is we know of no reference that lists the rod sizes. We have archived a list of the factory rods per the final blueprint specs from Carter Automotive Company. The specs compiled here provide the first comprehensive reference that we know of. These specs give the rod diameter at a glance, to make rod selection and tuning easier. A fatter rod plugs up more of the jet, reducing cross-sectional area, and thus fuel flow. Most TQ rods have three steps: economy, in action when under light throttle cruise; mid, under higher load or throttle opening; and power, high load and WOT. Remember, as soon as the throttle is snapped to wide open, the rod will instantly be up on the power step, no matter the rpm. Primary jets are also numbered, but last three digits of the number relates to the orifice size directly. For instance, a primary jet 4095 is .095-inch or a 4101 measures .101-inch. Similarly, the last three digits of a secondary jet equates to the size as a fraction of an inch. A 5110 jet measures .110-inch, while a 5143 measures .143-inch.
After cleaning (a soak in...
After cleaning (a soak in a tub of old-style carb cleaner is best), the subassemblies are reloaded with their respective parts for final assembly.
Do not forget to install the...
Do not forget to install the O-rings or the carb will be horribly rich, and the car will be undriveable.
Some emissions TQs seal and...
Some emissions TQs seal and vent the bowls solely through this solenoid valve at the rear. Unless wired-up and energized, the bowl will not vent properly, creating a fuel-starvation condition. Leaving off the rubber seal will allow the carb to vent normally, even with a failed or disconnected solenoid, but defeats the emissions intent of the device.
On '75-and-newer passenger...
On '75-and-newer passenger car TQs, we find a flat boss at the front of the air horn, which may or may not have various holes drilled into it, often with confusing devices attached. Here's the deal: these carbs were equipped with an auxiliary air bleed circuit, and the devices at the front open and close this air bleed to the atmosphere. When open, the signal to the primary main jets is weakened, leaning the mixture. The setup normally runs open, and is controlled by vacuum to the closed (rich) position during cold start-up. Besides the vacuum control, some carbs carried a barrel-shaped bellows (shown here), which varied the opening in relation to air density, to compensate for altitude changes. A common TQ mod, the control valve can be blocked by replacing it with a fabricated plate (shown in the left foreground). Removing the control mechanism and blocking the air bleed results in the carb running at the rich setting full time. As we will see, this is not always the best way to go.
For a rolling test bed, James...
For a rolling test bed, James Schagel volunteered his 360-powered '75 Duster. The stock short-block 360 is fitted with an MP .455-inch lift camshaft, 360 smog heads upgraded with 2.02-inch intake valves, a Weiand two-plane intake, and stock exhaust manifolds blowing into a 2 1/2-inch system. It's a mildly modified street 360, which we felt typified a large group of street enthusiast's combinations.
Using a TD adapter, the TQ...
Using a TD adapter, the TQ was mounted to James' 360. Call us odd, but the TQ carb just looked right on that small-block Mopar.
Carter ThermoQuad 9134S Rear Wheel
Horsepower at STP SuperFlow
Chassis DynoTested at Westech
|RPM||Rebuilt TQ||SuperTuned TQ|
Fired up, the TQ was adjusted...
Fired up, the TQ was adjusted for idle speed and mixture, and ran perfectly. The first pull netted us 206 hp at the rear wheels with the tach clocking 4,500 rpm. It seemed a little weak, and a check of the timing showed 25-degrees total.
James provided an acceptable...
James provided an acceptable explanation for the wildly maladjusted timing by claiming to having retarded it for a smog inspection, nearly always a sharp move to burn down hydrocarbon readings. We read the light, while James clocked the dizzy to a more appropriate setting of 36-degrees total. Power snapped up to 225 hp at 5,000 rpm. That was more like it. Wide-open throttle air/fuel ratio showed a rich reading, pegging mid-10s to low-11s on the scale. We already had the stock jetting: .092 primaries and .110 secondaries, about as lean as TQ jets come. Interestingly, power was already equal to the level reached by James' previous aftermarket carb. Since the carb already had what were considered quite lean jets installed at both ends, we looked in other areas to trim the fuel curve before cracking the carb open for a jet or rod change.
The secondary mixture of a...
The secondary mixture of a TQ carb is affected by the wide-open position of the secondary air door, as this varies the pressure drop on the discharge nozzles. The general base setting here is .500 inch as measured from the rear of the choke horn to the edge of the air door held wide open. There is some leeway here to set the door at greater wide-open position to lean the mixture.
Bending the ear on the air...
Bending the ear on the air door where it meets the stop, adjusts the air door wide-open position. We opened the door to .550 inch and found the curve began considerably leaner, but ultimately returned to about the same ratio by the top of the pull, just into the 11s. Power was not improved.
At WOT, we were fatter at...
At WOT, we were fatter at high rpm and leaner at low rpm than we felt would be optimal. Getting more fuel at the front of the curve would be easy to adjust with rod/jet and air door position. We wanted to get the top-end of the curve close by other means before jetting, since the secondary jets were already the leanest commonly available size, at .110. Our next move was adjusting the float level. The float level has a significant affect on mixture, and though we had set this carb to the 2 7/32-inch level listed for stock application, a lower 1-inch spec was common for earlier carbs. Changing the float level to 1 inch leaned the mixture across the board, giving us a much trimmer mixture at high rpm, reading just under 12:1 at the top of the curve. With the lower float level, we were getting closer up top, but close to critically lean at lower rpm. To compensate, the air door wide-open position was returned to a normal .500-inch setting. With these changes, we had our best pull yet, recording 228 hp at 5,100 rpm.
It is commonly believed the...
It is commonly believed the secondary is so disproportionately large, that WOT mixture is negligibly affected by what is going on in the primary. To put that theory to the test, the auxiliary air bleed block-off plate was removed. Interestingly, we found the mixture was leaned out substantially throughout the WOT pull, with our high rpm ratio now moving to the mid-11s, about one-half a point leaner. Power was up to 226.4 hp at 5,100 rpm. The lower rpm mixture was affected even more, leaning the mixture 1 1/2-2 points, into the 14:1 range from the 12:1 range. We were a little too lean at the beginning of the pull and still fatter than we wanted to be at the top of the curve. Toying with the air door wide-open position setting further, to .585-inch, resulted in an even leaner condition down low, and about the same up top. The conclusion here was that air door position primarily trims the front end of the fuel curve.
We wanted to trim the WOT...
We wanted to trim the WOT fuel curve further, taking even more fuel from the top. We pulled the metering rods and swapped from 1996 rods to 1966 size. This rod is mildly richer at cruise, but substantially leaner at full throttle (see metering rod chart). The rod change had the desired effect, recording a ratio of 12.5:1 up top. Power was up again with the change to the tune of 229 hp. While tuning for an optimal full-throttle fuel-curve may seem complicated, add in driveability considerations and transient response, and the tuning process becomes infinitely more complex. We determined that an even leaner WOT setting would help peak output, but running the car at part throttle showed it was running at the ragged edge of a lean miss. We needed more fuel at cruise, and were looking to lean WOT still further. A jet change alone would make the curve fatter or leaner everywhere, so here's where a jet/rod combination change can really dial in the mixture. We determined that upping the jet from the present .092 to a .095, while making a rod change from the present 1966 rods to a pair of 2145's would dial in about 7 percent more jet area at cruise, and reduce the WOT jet area by 6.7 percent. This move was aimed at reducing the fuel at WOT, while getting us out of a lean cruise condition. It worked on both counts, smoothing the part throttle driveability under light load, and posting a sizzling 233 hp at 5,100 rpm.